A porous sintered substrate for an electrode used in an alkaline storage battery is conventionally produced by sintering a metal plate with slurry in a reducing atmosphere including hydrogen. The metal plate, composed of iron or the like, constitutes a core of the porous substrate. The slurry, including a metal powder such as a nickel powder and a resin, is applied to the metal plate, and dried before sintering.
The porous sintered substrate is immersed in a nickel nitrate solution to fill nickel hydroxide as an active material into pores in the substrate by a chemical method or an electrochemical method. The chemical method is also called a chemical impregnating method, in which the porous sintered substrate is immersed in a nickel nitrate solution of high concentration, followed by drying and immersing in an alkaline solution to change the nitrate in the substrate into the nickel hydroxide. Several repetitions of the above operation are usually required to obtain a sufficient amount of nickel hydroxide.
The electrochemical method is also called an electrodeposition impregnating method, in which the porous sintered substrate is immersed in a nickel nitrate solution of high concentration, followed by cathodic electrolysis of the substrate in the solution. The electrolysis reduces a nitrate group in the pore within the porous substrate to generate an ammonium ion as shown in a reaction formula [1], resulting in a rise in a pH value. As a consequence, nickel hydroxide is precipitated in the pore within the substrate as shown in a reaction formula [2]. EQU NO.sub.3.sup.- +9H.sup.+ +8e.sup.- .fwdarw.NH.sub.4 OH+2H.sub.2 O[1] EQU Ni.sup.2+ +2OH.sup.- .fwdarw.Ni(OH).sub.2 [ 2]
However, a nickel nitrate solution of high concentration corrodes a sintered nickel body in the substrate in the chemical or electrochemical method because the solution is highly corrosive. Such a fragile electrode makes a cycle property of the alkaline storage battery worse.
For improving such a deterioration of the electrode, Japanese Patent Publication (Tokkai-Sho) No.48-100627 discloses a protective layer formed on the sintered nickel body. The protective layer is composed of nickel silicate, nickel phosphate or nickel carbonate. Japanese Patent Publication (Tokkai-Sho) No.59-78457 discloses a nickel oxide layer on the sintered nickel body, formed by treating the sintered substrate with a gas including oxygen. Japanese Patent Publication (Tokkai-Sho) No.62-61271, Japanese Patent Publication (Tokkai-Sho) No.63-128555 and Japanese Patent Publication (Tokkai-Hei) No.4-75255 disclose a cobalt oxide layer as a protective layer on the sintered nickel body. According to these publications, the cobalt layer can be formed by pyrolyzing a cobalt salt.
The protective layers disclosed in the above publications can give high resistance to corrosion so that the layers may be effective to prevent the corrosion of the sintered nickel body. However, the layers have low electric conductivity so that the utilization rate of the active material falls. In the case of the electrochemical method, nickel hydroxide is not precipitated on the protective layer because of its low electric conductivity but clustered only on the nickel surface that appears by fracturing the protective layer partially while carrying the substrate or operating. Moreover, the electrolysis rate (a rate of an amount of deposited nickel hydroxide to an input of an electric charge) drops due to the protective layer.